text:
When buying airliners what is the difference between "orders", "options" and "purchase rights"?
I've often seen announcements by aircraft manufacturers or operators along these lines "Qantas announced today that they will buy 10 A380's and have taken options on another 10".
What does it mean to "have an option"? I mean what level of contractual obligation exists between the manufacturer and the operator? Has a non-refundable deposit been paid - if so what sort of proportion of the eventual cost?
Does "having an option" mean much more than the manufacturer hopes you'll order those planes and has pencilled you in on the manufacturing schedule?
I also came across this on Virgin Atlantic's website:

Virgin Atlantic announced in March last year that it was ordering 15
  of the 787-9 Dreamliners - with options on ordering another eight
  787-9s and purchase rights on a further 20 aircraft.

I've never heard of "purchase rights" so I'd be interested to know about the significance of "purchase rights" as well.

Manufacturers typically plan their aircraft production well into the future, which includes having a buyer lined up for each aircraft, so they will know their position in the production line and have an anticipated date for delivery.
An order means that the buyer has agreed to purchase a certain aircraft in the production line, at a certain price. The buyer may cancel, but there will probably be a substantial penalty for doing so.
An option means that the manufacturer has reserved a certain aircraft in the production line for the buyer, usually at a certain price. However, the buyer will make the decision on whether to convert this to an "order" at a later date. There is less commitment than with an "order".
A purchase option is even less of a commitment than an "option." This just means that the buyer has agreed to purchase aircraft at a certain price, but it's not tied to a specific aircraft at that time.
It is becoming more common for buyers to mix these three types of orders, which allows them to have a balance of locking in an aircraft and price with orders, and less risk and commitment with options and purchase options.
These can all be canceled, though of course there will be more penalties for breaking a more firm commitment. Buyers may also change their order, either for their own or the manufacturer's business reasons.
The detailed finances of how much is actually paid for aircraft, and how much it costs to make or break any of these contracts, is generally not disclosed. Negotiations can be very specific to each order. For a rough estimate, this book considers that for 6 aircraft worth 45 million per unit, options would be worth 2.3 million each to the airline, or about 5%. This book estimates the down payment on a firm order at 10 to 15 percent, but notes that at least in the case of business and commuter aircraft, depending on market conditions, this may not always be required.

It is important to mention that "reserved a certain aircraft" means a position in the queue and consequently an (approximate) delivery date. The "reserved" aircraft does not exist yet; it is the n-th aircraft they will build. That's why it's worth having an option and why they are often traded.

Just to add, that the actual order is also sometimes called a firm order; while the reservation of an aircraft that will be produced is called an option.


text:
Which light aircraft are known to have a quiet cockpit?
Are some light aircraft known as being significantly quieter in the cockpit than others ? How quiet might the quietest light aircraft be from the inside ?
At least ten years ago I was reading an aviation magazine and there was an article about someone who had gone all out to make their aircraft cockpit as quiet as possible. I can't remember the details but they'd made a whole lot of modifications with sound reduction in mind.
Ever since then I've wondered: are some light aircraft really noticeably quieter than others (and do they get sold on the basis of them being quieter) and is modding your aircraft to make it quieter a common thing ?
(I am of course referring to closed cockpit aircraft, I appreciate a Gypsy Moth would inevitably be quite noisy !).

there are typically 2 sources of noise: engine noise, and wind noise. You can dampen wind noise quite a bit (and engineers do as a side effect of minimizing drag), but not much can be done about the engine.

@ratchetfreak Put the engine(s) as far from cockpit as possible.

@Farhan except the noise/vibration transmits through the structure, unless you make all oscillation outside the hearing range and ensure no harmonics occur inside it you will always hear the engine

Gliders are extremely quiet!

Its important to note that the dB scale is not linear, its logarithmic. This means that every 10 dB is 10x louder. So an airplane with a cockpit sound level of 90dB is 100X louder than one with at 70 dB. Let that sink in. Even just a difference of 10 dB (like 90dB vs 80dB) means it is 10X louder, (NOTE, not 10% louder, but 10 times more noise) as in, 1000% more noise.  See https://www.noisehelp.com/decibel-scale.html

Bah. If there is any science to connect loud noises to hearing loss, I have not heard it...

I assume that you're asking about piston engine light aircraft, so we can ignore gliders and electric engines for now.
The basic point is that they're all loud. One study compared a Cessna 172 (single engine) and a Piper Seminole (twin engine) and found that the average cabin noise was around 86 decibels, with no huge difference between the two aircraft. There's more analysis of a Seminole here. The FAA says that a light aircraft cabin is in the range of 70-90dB and this Cirrus owner measured 92-98dB in different Cirrus models. OSHA in the US requires ear protection and other measures for workers if the noise in their environment is 85dB or more.
Since decibels are on a logarithmic scale, the difference between 86dB and 98dB is large. But I searched a few aviation forums and there's very little discussion of cabin noise in light singles apart from a few subjective remarks. I couldn't find any good sources that said one aircraft type is well known as being noisier or quieter than average. The only consistent comment is that helicopters are louder than fixed-wing aircraft, which is also shown in the FAA document linked above.
Having said that, there are quite a few articles online about soundproofing aircraft in different ways. This one quotes the example of a jet:

Aero Sound Shield is currently soundproofing a particularly large
  corporate jet - a converted McDonnell-Douglas MD-87 - for a Las Vegas
  hotel owner who is almost totally blind and who has compensated by
  developing extremely sensitive hearing. "He wants a 60-dB airplane,
  which is quieter than a Lexus," Nelson points out.

But the same article makes the point that modifying any aircraft to add soundproofing increases the weight to a point that makes the aircraft much less useful:

Unfortunately, your airplane and mine can't accept the mass and weight
  of MD-87 measures. "I rode one time in a Piper Arrow that was very
  quiet," Nelson recalls, "but it was completely lead-lined. It could
  carry two people and half a load of fuel. That was fine for that guy,
  because that's all he ever wanted to carry, but it wasn't very
  practical."

Furthermore, it's worth remembering that modern noise-cancelling headsets are very good so the problem of noise (as opposed to vibration, which is also a factor for fatigue and health) has been more or less solved for most people. Even if you spend $1000 or more on a high-end ANR headset, that's a minor cost compared to altering the aircraft structure or materials and you still have full performance and useful load. I guess that for most GA pilots that's the decisive factor.

Great answer. Thanks for the effort you put into it. I would love to try the "Lexus Jet" out !

I have owned a  76 Piper Warrior (160 HP), 76 Piper Archer (180 HP) and a  Piper Dakota (235 HP) .... I have also flown in Cessna 172, 182, 210, Mooney Executive, Bonanza A36.
The Warrior and the Archer Pipers had the "super soundproofing" option that Piper offered in the 70s... the Warrior and the Archer were both so quiet you never needed headphones and could carry on a conversation the same as if you were in a car traveling at 75 MPH on the interstate.
The Dakota did not have the super soundproofing and it was louder and I needed headphones. I installed a 1/4' thick windshield on the Archer and the first set of side widows and that reduced the then even quiet noise levels a lot... well worth the cost.
All of the other aircraft I listed that I have flown were much much much louder then the Warrior and the Archer, to the point I never would consider buying any of them.
If you want a quiet airplane try the Archer or a Warrior but they no longer make the Warrior (dated 2017).

One thing that's alway puzzled me is why airplane engines never seem to have effective mufflers.  Is it just the Harley-Davidson effect - noise = power?

I recall the Pipers being MUCH quieter than Cessnas, especially the C15x which are deadening. I don't think the ones I was in had the super soundproofing. Anyone here compare with a Commander 112/114? They are generally commented on as being roomy and quiet.

From what I can remember of what I've flown, the Cessna 182 was far more quiet/smooth than the 172/other lighter trainers. No clue what the quietist aircraft is, or how nice you can get it. I'm not aware of any aircraft being marketed on it's quiet features.

Interesting, thank you.

I can imagine why the 182 would be more quiet than the 172 when used at the same speed and altitude. It's a slightly larger aircraft (distance between cabin and engine a bit larger, plus maybe less vibrations), and the engine would run at a lower power setting, thus probably also more quiet. Run both at max power and I think the difference goes out the window.

182 is larger and has higher gross weight, which could allow for more insulation reducing cabin noise. I don't know if it actually has more, just a thought.

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text:
What are the pilot responsibilities for "maintain visual separation" clearances?
ATC will sometimes give pilots the instruction to maintain visual separation from nearby traffic.
I have a couple questions about this:

There is a related question here about traffic separation.
Is the required separation in this case different, and how can they judge this? (I would like a bit more detail for this specific case than was provided on the answer there)
Normally pilots will follow their flight plan and ATC instructions. What actions should pilots take to maintain separation? I mean, obviously whatever is necessary, but is there any standard method? Should they anticipate an issue and ask ATC if they need to change course?

This is mostly focused on IFR traffic, but how different would VFR be? The situation could be that ATC clears you for a certain maneuver, and other traffic interferes.
It is my understanding that for IFR traffic the controller would expect that there would be no conflict, but things don't always go as planned.

Visual separation is addressed specifically in the AIM (see below).  It is an ATC instruction sometimes given when the pilot reports another aircraft in sight.   ATC is then able to instruct the pilot to "maintain visual separation" from the other aircraft, and it doesn't matter if they are IFR or VFR.
Then it becomes quite simple:

Don't hit the other airplane!

Do whatever you have to do in order to avoid them.  (Usually this is nothing since you just get closer than ATC is allowed to let you otherwise.)
If you are on a specific ATC clearance, then notify ATC immediately if you have to do something contrary to that clearance.

Keep the other aircraft in sight.

Immediately report to ATC if you lose sight of them.

Avoid the wake turbulence of the other aircraft.

Standard radar separation no longer applies since the pilot has accepted separation responsibility from the one specific aircraft that the clearance applies to.  This allows ATC to let aircraft fly more efficient routes (not having to turn to avoid the other aircraft) and also lets more aircraft land in a given amount of time because they can be closer together on final.
Pilots have to use good judgement as to how close they get to the other aircraft, but it is done visually.  They can use any tools at their disposal to help (TCAS, asking ATC how far away they are, etc.) but it all comes down to "that looks good enough for me".

Section 5. Pilot/Controller Roles and Responsibilities
5-5-12. Visual Separation
a. Pilot.
1. Acceptance of instructions to follow another aircraft or to provide visual separation from it is an acknowledgment that the pilot
  will maneuver the aircraft as necessary to avoid the other aircraft or
  to maintain in‐trail separation. Pilots are responsible to maintain
  visual separation until flight paths (altitudes and/or courses)
  diverge.
2. If instructed by ATC to follow another aircraft or to provide visual separation from it, promptly notify the controller if you lose
  sight of that aircraft, are unable to maintain continued visual
  contact with it, or cannot accept the responsibility for your own
  separation for any reason.
3. The pilot also accepts responsibility for wake turbulence separation under these conditions.
b. Controller. Applies visual separation only:
1. Within the terminal area when a controller has both aircraft in sight or by instructing a pilot who sees the other aircraft to
  maintain visual separation from it.
2. Pilots are responsible to maintain visual separation until flight paths (altitudes and/or courses) diverge.
3. Within en route airspace when aircraft are on opposite courses and one pilot reports having seen the other aircraft and that the
  aircraft have passed each other.

More info is here:

4-4-14. Visual Separation
a. Visual separation is a means employed by ATC to separate aircraft in terminal areas and en route airspace in the NAS. There are
  two methods employed to effect this separation:
1. The tower controller sees the aircraft involved and issues instructions, as necessary, to ensure that the aircraft avoid each
  other.
2. A pilot sees the other aircraft involved and upon instructions from the controller provides separation by maneuvering the aircraft to
  avoid it. When pilots accept responsibility to maintain visual
  separation, they must maintain constant visual surveillance and not
  pass the other aircraft until it is no longer a factor.
NOTE-   Traffic is no longer a factor when during approach phase the other aircraft is in the landing phase of flight or executes a
  missed approach; and during departure or en route, when the other
  aircraft turns away or is on a diverging course.
b. A pilot's acceptance of instructions to follow another aircraft or provide visual separation from it is an acknowledgment that the
  pilot will maneuver the aircraft as necessary to avoid the other
  aircraft or to maintain in-trail separation. In operations conducted
  behind heavy jet aircraft, it is also an acknowledgment that the pilot
  accepts the responsibility for wake turbulence separation.
NOTE-   When a pilot has been told to follow another aircraft or to provide visual separation from it, the pilot should promptly notify
  the controller if visual contact with the other aircraft is lost or
  cannot be maintained or if the pilot cannot accept the responsibility
  for the separation for any reason.
c. Scanning the sky for other aircraft is a key factor in collision avoidance. Pilots and copilots (or the right seat passenger)
  should continuously scan to cover all areas of the sky visible from
  the cockpit. Pilots must develop an effective scanning technique which
  maximizes one's visual capabilities. Spotting a potential collision
  threat increases directly as more time is spent looking outside the
  aircraft. One must use timesharing techniques to effectively scan the
  surrounding airspace while monitoring instruments as well.
d. Since the eye can focus only on a narrow viewing area, effective scanning is accomplished with a series of short, regularly
  spaced eye movements that bring successive areas of the sky into the
  central visual field. Each movement should not exceed ten degrees, and
  each area should be observed for at least one second to enable
  collision detection. Although many pilots seem to prefer the method of
  horizontal back-and-forth scanning every pilot should develop a
  scanning pattern that is not only comfortable but assures optimum
  effectiveness. Pilots should remember, however, that they have a
  regulatory responsibility (14 CFR Section 91.113(a)) to see and avoid
  other aircraft when weather conditions permit.

For even more information, there is an entire section of the Air Traffic Control job aid which the controllers use about visual separation.

Ad "asking ATC how far they are", the radar scope has limited resolution. One of the "Say Again?" articles mentioned the "blip" is about a mile across so at that range the controller won't even know which aircraft is on which side.

@JanHudec I tend to do that more with approach control while on final, so they usually have better resolution than that.

FAR 91.111 also applies: "No person may operate an aircraft so close to another aircraft as to create a collision hazard."

ATC will use radar to judge how far apart planes are, in absence of ATC (uncontrolled airspace) it is purely scanning the sky and paying attention this is called See and Avoid and is always in force even during IFR.
If a pilot notices he is on a collision course with another plane he may do anything in his power to avoid said collision. For many planes there is TCAS an automated system to help pilots avoid collisions by issuing a climb or descend command to the pilot if collision is near. If such a command happened then the pilot MUST follow it to minimize chance of collision and notify ATC.

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text:
What is TAPS in this report?
In researching for this question, I came across some reports from EASA referring to TAPS. What is the meaning of TAPS in this context?

TAPS means Twin Annular Premixing Swirler, a cyclone mixer / combuster designed to keep the NOx emissions of the General Electric GEnx engines low.

source: FAA report
AIAA report


text:
How do airports handle traffic during emergencies?
What does ATC do when there is an emergency? This could be a tower or an ARTCC being evacuated or otherwise unusable.

How do they decide whether to close the airport/airspace?
What do they do with the traffic, whether they do or don't close?

On this related question, it turned out that Newark closed because of smoke in the tower.
Another user posted an interesting anecdote about another tower being evacuated, so I thought it warranted a question.

The facility under duress will declare "ATC Zero" and all traffic will be handed off to other controllers while that facility is offline.
In the case of a control tower, this means the airport either acts as a non-towered field or as a closed field (depending on the nature of the emergency).  If it is a TRACON, then the ARTCC will assume control of the TRACONs airpsace.  If it is an ARTCC, that might present bigger problems as continuity of radio communications may be impacted and I'm not sure exactly how this would be handled but assume it would be re-routes around the ARTCC.
I've been inbound to IAH when their tower decided to close (I forget why), and the response was to put everyone inbound to IAH into holds and handle diverts when fuel became an issue.

In the U.S., the facility will execute its OCP (Operational Contingency Plan) which is governed by FAA Order JO 1900.47C. Each facility must develop its own specific plan per these guidelines, and practice the plan at least once a year (unless the plan was actually used for real that year). (Making this a comment instead of an answer since yours is already quite good; feel free to incorporate the reference if you wish.)

I happened to arrive in the Bridgeport, CT tower for a tower tour precisely when the boss (offsite) called an evac drill. KBDR is a non-radar tower. The two people in the tower picked up battery powered radios and evacuated to the parking lot, and continued controlling traffic from there. The controller on duty had/has a mental picture of the traffic and (at that low volume airport) he could operate just as well from the parking lot as from the tower.

How do they coordinate with other facilities in that case (IFR arrivals / departure handoffs)?

@dvnrrs - I don't believe they have ILS at Bridgeport, so there probably isn't much call for inbound IFR flights, at least.

@Jon Story:  Sorry, not true.  Runway 6-24 has an ILS DME approach.

I stand corrected :) I'd be interested to see the answer, then

I don't know the answer, but most likely the traffic is sent to a holding pattern or diverted.


text:
Is there an engine thrust level limit for deploying thrust reversers?
Does an engine have to be at idle thrust for the reversers to deploy? If the engine is at higher than idle thrust, will it slow down to deploy the reversers and then speed up again, or can it just deploy the reversers at the higher thrust?
I am also asking about turboprops going into beta range.
I had this question based on the discussion on this question:
What to do when you accidentally land on a runway thats borderline long enough to land on?

Engines must be at idle thrust to enter reverse thrust.  This may not be 100% true of the physical action of engine entering reverse thrust (opening the buckets or translating the cowling), but it is required by the thrust levers.  In all airplanes I'm familiar with the thrust levers must be at idle to enter the reverse regime,  In the EMB-145 the reversage was below idle (i.e. you keep moving the levers aft) through a gate.  Even if the engine is capable of entering reverse thrust not at idle, there is no way to command it without first being at idle.

Yep, worst case is that you quickly retard the throttles from a higher setting to idle and then into reverse, and the engine hasn't spooled down completely by the time the buckets come out (or whatever). Some planes may have some kind of protection system that monitors N1 but I don't know of any specifics there.

On 747-100 and -200 aircraft, each reverse thrust lever is mounted on the front side of it's respective thrust lever with the hinge above the reverse lever's end knob. If you should happen to grab the reverse thrust lever without having the forward thrust lever all the way back, the action of lifting the reverse thrust lever will force the forward thrust lever all the way back. When you lift reverse thrust lever to an almost-level position from it previous position parallel to the forward thrust lever, the buckets come out. That in itself provides considerable braking, and it's common to stop there. Continuing to lift and then bring back the reverse thrust levers spools up the engines. You need to be careful doing this. Coming back too far too fast might cause a compressor stall, especially on an already marginal engine. Compressor stalls on a Pratt and Whitney engine are no big deal. On a General Electric engine they are far more serious. As I remember, there was a lock-out that prevented an engine from moving into reverse thrust from other than idle thrust, but I can't remember how it worked, but I believe it required not just idle thrust but the aircraft also being on the ground (squat switches) and landing flap deployment.

Why are compressor stalls so much more serious on GE engines?

@Sean I don't know the answer to that. Of the 23 747s the airline I retired out had at the time of my retirement, only one had GE engines, and I don't recall ever hearing of a compressor stall on it. All I can tell you is that we didn't write up compressor stalls on P&W engines, but were instructed to write up a GE compressor stall, which would mean maintenance would have to take a look at the engine.


text:
Could supersonic transport make a comeback?
When the Concorde was retired from service, reasons given included:

It was expensive to operate; it burned a lot of fuel and had high maintenance costs, resulting in high ticket prices
Could only operate on overseas routes where the sonic boom it created could go essentially unnoticed
It had a very limited market because of the above

But the Concorde was built with 1960s technology and aeronautical science. Modern aero engines are more fuel efficient, modern materials and manufacturing methods could potentially reduce maintenance costs, and there is today about 50 years of additional aeronautical science available.
So my question is this:
Is it possible to design a 200-300 seat airliner having performance comparable to Concorde (Mach 2+ at 50,000 ft or so), without creating a noticeable or objectionable sonic boom at ground level, and operating economics at least close to that of a typical subsonic airliner?
I have read similar questions ([one] and [two]) which explore the reasons Concorde was retired, but the discussions are framed around Concorde itself - an aircraft developed in the 1960s and never updated, or the abandoned Boeing 2707. None appear to acknowledge that at least four decades of scientific and technological progress has occurred since these aircraft were in development, and how that may or may not alter the feasibility of an SST service flying an all-new design. They also don't address the question of  whether new advances allow for large aircraft to fly supersonic without creating a noticeable or objectionable ground-level sonic boom.

related: https://aviation.stackexchange.com/q/3782/1467

also related and possibly duplicate: https://aviation.stackexchange.com/q/29204/1467

The question is how much more money people are willing to pay for increased speed. Take railway for example, Beijing-Shanghai is around 800 miles where for equivalent of 75USD you have the choice of 4.5 hour on a coach seat on a high speed train or 14 hours for a sleeping car on a regular express train. Most people pick comfort over time efficiency. Even worse, right now IMO time sensitive business travel is already replaced by information technology.

While the other questions mention the Concorde, personally I don't see how the answers would not apply to any other project in the same category, since they are general answers that do not deal with any specific project or airframe.

@user3528438: Another reason is diminishing returns.  If you look at total journey time, a large fraction is spent in not flying (especially not at cruising speed), but on TSA security theater, boarding, taxiing & waiting for takeoff, holding in landing patterns, deplaning & waiting for luggage...  Flying at supersonic speed doesn't really gain much for an airline passenger, though of course a supersonic bizjet could bypass much of it.

@jjack if we have to speak about the future, then it risks becoming opinion based, because we cannot forecast what will happen.

@jjack that's opinion, and the SE network considers that as off-topic.


text:
How much would it cost for a large airline to switch engine manufacturer for their planes?
I am trying to understand how much of a hassle and cost it would be for a large civil airline, who already had lots of installed Rolls Royce engines on its fleet, to switch over to GE engines on new planes they have ordered. For example if they opted for the RR Trent 1000 on a 787 initially but then changed new orders of 787's to have the GEnx-1B.
Very intangible I know, but would it be fair to say that switching would require some difficulty including:

retraining of pilots
changing maintenance schedules
additional uncertainty over performance and fuel efficiency

Alternatively, I could be wrong, and the airline will likely just switch to the GE if it thinks it can gain a small fuel burn advantage, in which case RR's back book of engines does not mean clients will opt for the RR engine choice going forward.

if the switch is made after the initial contract is signed then the RR engines may have already been ordered by the constructor and would need to be canceled (with all due costs)

And what about for new orders, where contracts have not been signed yet? Is an airline likely to change engine provider?

@Ben most airlines have a history and infrastructure for engines from a specific manufacturer and are unlikely to change that, the cost would extend far beyond just the cost of changing the contract. E.g. KLM has a massive investment in maintenance facilities for GE engines, workshops, spare parts warehouses, highly trained specialised technicians. Retooling to RR would be prohibitive for them.

Many (especially larger) airlines have a history and infrastructure for engines from a specific manufacturer and are unlikely to change that, the cost would extend far beyond just the cost of changing the contract. E.g. KLM has a massive investment in maintenance facilities for GE engines, workshops, spare parts warehouses, highly trained specialised technicians. Retooling to RR would be prohibitive for them.

Of course a smaller operation without maintenance facilities of their own might be more flexible in that and just buy whatever is cheapest at the time, contracting out maintenance to various companies as needed. But even then, it's often more economical to have a single maintenance provider doing all your work for you, which might well mean a provider who's specialised in a single series of engines, for example if you're based at Schiphol there's a good chance you'll be contracting out your maintenance to KLM and their GE specialised shops.

So there are many good reasons for airlines to stick with a single engine supplier (and to a degree with a single aircraft manufacturer), even if the cost of buying those engines is higher than selecting competing engines from another manufacturer.
Say you're going to save 0.1% in your fuel bills for the 2 new aircraft you're buying to increase the fleet of 20 with other engines you have already (and that's a big if, those engines are pretty close between manufacturers for a specific aircraft). You'd need to fly a LOT for it to be worth building an entire duplicate maintenance network just for those few new aircraft.
Of course if it's part of a long term plan to replace the entire fleet with aircraft using that other manufacturer's engines, that's a different story. But mixed engine fleets tend not to be the most economical way to go, even if on one type manufacturer A is marginally more fuel efficient and on type B manufacturer B is marginally more fuel efficient (and that's what you're going to see, marginal differences only).
If you're talking an operation like Delta or United, with hundreds of aircraft of each type, it might be a different story. But then too you're talking about more aircraft with each manufacturer's engines than most other airlines have in total, and they'll have multiple maintenance shops themselves, making it easier to have a facility for each manufacturer (and of course in case of the big US airlines, they tend to have ended up with that in the course of gobbling up other airlines anyway).

There are obvious costs in having a mixed fleet. Particularly in engineering support infrastructure. But large airlines have less of a problem
There must be some negotiating advantage in having a mixed fleet, when buying engines the maker knows you can switch more easily than an all-GE airline or all-RR airline.
Take as an example BA, with a fleet of 290 aircraft
Switching

BA's first 777s were fitted with General Electric GE90 engines, but BA switched to Rolls-Royce Trent 800s for subsequent aircraft.

Wikipedia
Have an engine maker maintain it's rival's engines?

British Airways has agreed to a $2.3 billion engine maintenance contract with General Electric  ... Under the 10-year agreement, GE will be responsible for the maintenance, overhaul and repair for British Airways' RB211-524 and CFM56 engines. The Rolls-Royce RB211 engines power the airline's 57 Boeing 747s and 21 767s. The CFM56 engines power the airline's 34 Boeing 737s and 10 Airbus A320s.

http://www.geaviation.com/press/services/services_20051208.html
Share costs with other airlines?
Airlines can also share maintenance facilities, which is a way of reducing maintenance costs.

Since January 2012 most heavy maintenance on the Boeing 737-400 Gatwick-based fleet has been done at the Lufthansa Technik maintenance facility in Sofia, Bulgaria

Wikipedia
Other large airlines

AA have a mixed fleet of 627 aircraft with  both GE and RR engines.
UA have a mixed fleet of 700 aircraft with both GE and RR engines

Conclusion
I think you'd find it hard to come up with a specific figure for switching between GE and RR because it depends very much on arrangements any specific airline has in place (e.g. wet-leasing) and on the terms it is able to negotiate with engine makers and maintenance organisations.

Switching cost depend a lot on the airline and how they handle their relations with the engine manufacturers. For a well-run airline, switching cost are near zero.
Big airlines have an interest in keeping a healthy competition alive, and they have an advantage in running engines from all the big engine manufacturers. If all aircraft in the fleet would have engines from the same manufacturer, the airline's negotiating power for future engines would be quite limited. If, on the other hand, they have experience with several manufacturers and could be equally likely to buy from either, they can expect to negotiate steep discounts from all of them. Normal discounts are already 30%, but the real prices are confidential.
The same goes for airframes, that is the reason why most airlines have a mix of Boeings and Airbuses. Some have political constraints, but the well-run airlines try actively to avoid a monopoly. Exceptions are/were JAL, who are under pressure to buy American only, or in the past Swissair, which had only McDonnell-Douglas aircraft, because the airline was heavily staffed with ex Swiss Air Force pilots who had an unhealthily close connection with that manufacturer.
This is nothing airline-specific, but a basic principle for any good purchasing manager. Manufacturers try to drive switching cost up, and the customers try to drive switching cost to zero.

@David Richerby: You are right, that was a bad example. I only selected them because they had until recently not bought Airbus, even though the Airbus wings are made in Britain.

They've flown Airbuses for 26 years, which seems quite a long time to me. Anyway, not a big issue. Nice answer.

@DavidRicherby: Thanks for the correction and helping to improve the answer!


text:
What does “Ground loops are costly. Fly them until they stop” mean?
In one of the earliest scenes of William A. Wellman's 1942 film Thunder Birds, a sign can be seen in a room of a USAAF base (some relax or briefing room, apparently), which says:

Ground loops are costly. Fly them until they stop.

I am not entirely sure about its meaning. I know what a ground loop is, but in what sense “Fly them until they stop”? Is it an advice not to use brakes too soon after landing?

@BenCrowell: The matter is explained by a voiceover at the start of the film. In that base American, British and Chinese pilots were trained.

It just means "pay attention to what you are doing" until you are no longer moving.  "Fly them" means "keep actively controlling the plane as if you were still in the air". Don't start daydreaming just because you're on the ground, in other words.
Especially with the taildraggers of olden times;

They are dynamically unstable turning-wise while rolling along the ground (because the CG is behind the fixed traction points; try driving a car backward really fast) and require careful attention to staying directionally aligned even while moving slowly.  Taildraggers with locking/free swivelling tailwheels (which you tend to see on the larger ones) are even more demanding, and are basically like a shopping cart rolling backwards (the kind with fixed back wheels, not the 4-wheel caster kind) once the tailwheel is unlocked for taxiing, and need to be steered with brakes alone, which requires quite a bit of concentration.
They like to tip forward under hard braking so keeping full up elevator while taxiing is a requirement to minimize this tendency
Strong quartering winds, if present, are trying to tip you over while 3 point on the ground and you have to learn to apply appropriate amounts of aileron to counteract this tendency.

A lot of stuff to keep in mind.  The tendency of a new pilot, when they had a successful landing, was to breath a sigh of relief and relax during the rollout when dropping below, say, 30 kt where rudder power fades away, or during taxi, forget they were in a backwards rolling shopping cart that is trying to switch ends continuously and let their mind wander, and oops.
Tricycle gear airplanes are nowhere near as physically demanding, but still you need to pay attention to what you are doing so it still applies in the end.

Note that even tricycle-gear aircraft are usually still easy to intentionally ground-loop; they just tend not to do so spontaneously.

This very good answer could be further improved by noting that groundloops (at least in tailwheel aircraft) often tend to begin once the airspeed has decayed to a rather low value.

@Sean not really.  It's hard to make a trike actually spin around unless you are on an icy surface.  It simply can't switch ends the way a TD can.  It's the shopping cart going FORWARD; push it along and let it go and it just wanders off to one side. Inattention on a tricycle gear plane just normally means drifting toward the runway lights, still an exciting event all the same.

@quietflyer good point.  Added a bit of text.

The basic problem with a taildragger is that the c.g. is behind the main gear as opposed to a tricycle gear aircraft, where the c.g. is in front of the main gear.

The shopping cart analogy is a bit confusing when you, like me, are from a country where shopping carts generally have 4 free spinning wheels and going forward and backward is exactly the same. This answer refers to to shopping carts where the rear wheels are locked. (Common in the US I think, maybe in other places as well.)

@Maeher in Canada I've seen both but most are US style.  A 4 wheel caster shopping cart might be closer to a taildragger with "crosswind" landing gear, where the mains are able to caster within a limited range to let the airplane dogleg while rolling.  In any case, I've made an edit.  Thanks.

It might be useful to mention that "them" in the second sentence apparently refers to "airplanes". Not to "ground loops", which might be what someone might thing.

this actually makes a lot more sense than my aviation ignoramus interpretation of this being aimed at ATC, essentially meaning "it's expensive to have planes cycle on the ground. just let them fly until they crash, that's cheaper".

@user3067860 I thought "them" referred to the pilots. As in "If a pilot is doing ground loops, then that means they need more practice landing a plane, so have them fly until they get it right". This answer takes a lot as given (what "they" means, what ground loops are), and is less explaining that this means X, and more taking it as a given that the reader knows it means X, and explaining why people would say X.

@Acccumulation added some text at the top.

Ground loops are costly. Fly them until they stop.

What this literally says:

Ground loops are costly. Fly the ground loops until they stop.

What they meant to say:

Ground loops are costly. Fly the airplane until it stops.

Some background to why this was was important, not an answer to the question really...
From personal experience (twice) it is all to easy to relax and lose focus as soon as the airplane is rolling on the ground, the landing went quite well you think.
In a tailwheel plane you have to focus on directional control during the rollout. If there is a course deviation that is not corrected immediately it will quickly amplify out of control and develop into a ground loop. The situation is also complicated by a risk of pilot induced oscillation. If you correct a deviation to say the left and the tail is then swinging back to the right, you have to start stopping this right swing before reaching the neutral line otherwise you have a developing ground loop to the right that has to be stopped.
So, focus on the rollout, do nothing distracting like flipping switches, retracting flaps or folding maps until the aircraft has fully stopped.
The rollout is an important part of the flight.
Another saying: The flight is not over until it is over.

You could improve this answer by explaining what is meant by "fly" in this quote. It can't be about actual flying in the air, can it?

@Dzuris "Fly" in this context just means "it's an aircraft and you're in charge of it until you're stopped in the parking space with parking brake on/chocks under the wheels/tiedowns tied, engine off and checklists done."  It's easy to relax after the aircraft has touched down and appears to be rolling out straight only to have a gust of wind embarrass you, or do a great landing then be careless taxiing.  One is constantly admonished when practicing emergencies to not forget to "fly the plane."  In this case, "fly" means be "in charge of" - you're not finished until the plane is safely parked.

It means that even if you've put the wheels on the ground, you still need to "fly" the plane (meaning using control surfaces such as rudder/aileron/elevator) to maintain directional control until you've stopped.

It is amazing to realize how much aircraft changed from 1920 to 1940, and how difficult it was to properly train people to operate the new ones (all the way to stop).
A "ground loop" means exactly what it says, a very sharp turn from your intended path.  Your mechanic would not be happy when the cost of damage to struts, wheel bearings, tires, and other items (should a wing drag) is assessed.
The ground stability issues of these new planes came from 4 sources, greatly increased engine power, greatly increased weight, and, especially for this discussion, weight behind the brakes of the main wheels, and the natural "weather vaning" tendency of a directionally stable airplane.
The fourth is an issue of decreasing rudder authority in a crosswind rollout that has never changed. What did change was putting a wheel on the back instead of a skid, and the relatively larger vertical stabilizer on the newer low wing monoplanes as compared with biplanes.  Having a narrow track on the front, such as the Me-109, made the differential braking required to control this more difficult. Wider track mains helped.
Greatly increased power lead to much greater turning tendencies as throttle was applied, and the greater weight per area meant inertial "overshoot" with inadequate aerodynamic control became more of an issue (should be ringing bells for you "Dutch Rollers").
But the fundamental problem is brakes in front of weight.  Tractor-trailers are famous for "jack knifing" for the very same reason.  A fully loaded tractor trailer has 60% of its braking in front and 75% of its weight in back.  This is why it wants to "swap ends" when brakes are applied.  Very similar concept to drag and weight in the air (the parachute). Drag wants to be in back, weight in front for a decelerating object on the ground.
This conflicts with the need to have the mains "raked forward" to keep the plane from flipping forwards when landing!  Which brings us to the old term "tail dragger".  When older lighter planes landed, the skid in back would more stably drag it to a stop (from maybe 35 mph).  Newer planes, landing at 100 mph, with brakes on the mains, were a different story.
The tricycle gear greatly helps this issue because the mains are set behind the CG.
The "tail dragger" concept was successfully applied to the X-15, which followed the bi-plane tradition with 2 skids in the back and a wheel in the front.  The Space Shuttle was a trike.

What does “Dutch Rollers” refer to?

@DaG a plane that Dutch rolls.

but all aircraft "Dutch roll", it's one of the fundamental dynamic aircraft responses

@Federico yaw and roll coupling is fundamental, what we know as "Dutch roll" is considered undesirable.  This would be a sustained, repetitive, undamped cycle of movement once controlled inputs are removed.  Very difficult to get a paper airplane to do this .. why?

because you do not have movable surfaces on a paper airplane that can excite the motion. it is undesirable to have it excited, but the coupling, i.e. the dynamic response, is there

I actually found the paper delta "classics" rather spirally unstable in a cross wind, which lead to learning ways to balance the tendencies.

"maintain directional control until you've stopped."

In aviation, a ground loop is a rapid rotation of a fixed-wing aircraft in the horizontal plane (yawing) while on the ground. Aerodynamic forces may cause the advancing wing to rise, which may then cause the other wingtip to touch the ground. In severe cases (particularly if the ground surface is soft), the inside wing can dig in, causing the aircraft to swing violently or even cartwheel.

Thanks for your first sentence. The rest is from the Wikipedia article I myself linked to in my question.

GROUND LOOPS ARE EXPENSIVE ie if it gets away from you and enters a ground loop so severe that the aircraft flips and/or strikes a wing into the ground the damage both to the aircraft - and to you - will be very costly.
FLY THEM UNTIL THEY STOP - maintain positive control of the aircraft throughout all ground and flight operations.  This is particularly true of landing in crosswind conditions where the tendency is to release control pressures immediately after touchdown.  This quickly results in the airplane weathervaning in the wind and veering off centerline.  This is easily correctable on a tricycle gear aircraft with proper control inputs but with the inherently unstable nature of a conventional gear airplane can quickly get out of hand with overcorrections using differential braking and rudder, which may cause the airplane to enter an unrecoverable ground loop.  The remedy for this is to maintain the proper crosswind control inputs even after touchdown and fly the airplane until it is slowed to taxi speeds and ready to exit the runway.
You should maintain positive control of any aircraft with the correct flight control inputs during all phases of flight operations, but this becomes especially true in taildraggers due to their inherently unstable nature during ground operations. Tailwheel airplanes will quickly show you how bad your landing skills are and some like the Pitts S-2B will punish you for it!

From an "aviation culture" perspective, "fly it until it stops" is still a phrase taught to student pilots to this day.  It means "don't let go of the controls no matter what, act like you are in command of the aircraft until there is no more motion.". In modern times, this is taught in the context of landing in general, especially in emergency landing training (ground loops aren't a big concern with modern tricycle gear trainers).  Even when things are slightly out of control, even if the airplane is running in to things, a pilot can reduce damages and injuries by applying counteractions.  Just like in a car, if you skid, you don't let go of the wheel, you steer against the skid.
With this context in mind, "Ground loops are costly.  Fly them until they stop." means "ground loops damage the aircraft, if you find yourself in one, don't give up! Keep trying to regain control, until there is no more motion of the aircraft."

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How common are missile defense systems on civil airplanes?
I've just stumbled upon Why don't El Al 747's stay at the gates at JFK? in the hot topics.
I kind of get why El Al decided to put missile defense systems in their planes but I was wondering: How common is this? Are there other airlines who do this?

Obligatory:  I could tell you but then I'd have to kill you.

The day my inabillity to properly read a Wikipedia article made it to over 2k views...


text:
Is stall recovery training mandatory for Airbus pilots?
The recent reports about the Air Asia Flight 8501 crash mention that the plane was stalled and that the pilots could not recover from the stall. Indonesia's National Transport Safety Committee apparently had this to say:

Subsequent flight crew action resulted in inability to control the aircraft ... causing the aircraft to depart from the normal flight envelope and enter a prolonged stall condition that was beyond the capability of the flight crew to recover.

Online information that I could find suggests that Airbus pilots were not required to learn stall-recovery and other upsets recovery as part of training, due to the aircrafts "inability" to enter those conditions. This may have changed, at least for stall recovery, at some airlines, after the crash of Air France Flight 447.
Is stall recovery mandatory training for Airbus pilots? Was it ever not mandatory? How about for pilots of Boeing, Embraer, and other large passenger aircraft?

I read the accident report and it says the PIC was pushing down (correct response) while the SIC was pulling up (incorrect response). My guess is that the SIC was spatially confused; they were in bad weather after all. The computer adds the two inputs but the SIC was pulling stronger so he won out and the plane crashed.

The official 209 page report (in English) from the Indonesia National Transport Safety Committee http://kemhubri.dephub.go.id/knkt/ntsc_aviation/baru/Final%20Report%20PK-AXC.pdf

The official report says that training focuses on stall avoidance rather than stall recovery. But stall recovery is well taught in basic pilot training; just not in the Airbus 320

From reading the report, the problem was not that the pilots did not know how to recover from a stall (push the stick forward) but that the pilots - or at least the SIC - did not realize the aircraft was stalled - despite the stall alarms.

Stall training is mandatory for all pilots operating under CFR 121, albeit in a simulator. According to 14 CFR §121.423 Pilot: Extended Envelope Training,

(a) Each certificate holder must include in its approved training program, the extended envelope training set forth in this section with respect to each airplane type for each pilot. The extended envelope training required by this section must be performed in a Level C or higher full flight simulator, approved by the Administrator in accordance with §121.407 of this part.
(c) Extended envelope training must include instructor-guided hands on experience of recovery from full stall and stick pusher activation, if equipped.

FAA has not mandated in-aircraft in-aircraft stall or Upset Prevention and Recovery Training. ICAO's Doc 10011, Manual on Aeroplane Upset Prevention
and Recovery Training is pretty explicit:

On-aeroplane UPRT is not intended to be delivered while operating transport category aeroplanes or aeroplanes requiring two or more crew members; for those operations, UPRT should not be permitted to be conducted outside the confines of a suitable FSTD.

As a result of multiple stall related accidents ( including the Colgan Air Flight 3407, a Bombardier Q-400, Turkish Airlines Flight 1951, a Boeing 737 and Air France Flight 447, an Airbus A330 ), both the industry and regulators have taken a number of steps taken to improve the stall and upset recovery training. For example, Airbus has changed the operational documentation of almost all its aircraft, including A320 (via FCOM volume 3 Temporary Revision number 323-1 and QRH Temporary Revision number 727-1)
The basic template of the stall recovery training is as follows:

Immediately do the following at the first indication of stall (buffet, stick shaker, stick pusher, or aural or visual indication) during any flight phases except at lift off.

Autopilot and autothrottle.............................. Disconnect
a) Nose down pitch control... Apply until out of stall
  (no longer have stall indications)
b) Nose down pitch trim................................... As needed
Bank................................................................Wings Level
Thrust................................................................As Needed
Speed Brakes..........................................................Retract
Bank................................................................Wings Level

These have beeen incorporated in the FAA Advisory Circular 120-109A which changed the core principles taught to the pilots for recovering from stall. Still, the trainings are expected to be carried out in Flight Simulation Training Device only.

It is true that the pilots of Air Asia Flight 8501 were not given upset recovery training. However, this is according to the Flight Crew Training Manual and the philosophy that the aircraft's FBW system will prevent any such condition from occuring in the first place. From the report:

The flight crew had not received the operator upset recovery training on Airbus A320 as it was not required according to the Airbus FCTM.

and

The FCTM stated that the effectiveness of fly-by-wire architecture and the
  existence of control laws eliminate the need for upset recovery manoeuvres to be trained on protected Airbus.

Airbus sure is confident in their FBW systems, telling airlines they didn't need to train their pilots for upsets. Loss of envelope protections and pilots' improper response has crashed at least 3 of their FBW aircraft killing 397 people. I'm glad to see they finally backed off of their "no need to worry about that, it can't happen in our planes" philosophy.

According to the official report the PIC pulled the circuit breakers on the flight augmentation computer. This is completely crazy. The manuals state this action is not permitted in flight. Just because maintenance did it on the ground he thought he could do it in the air. So if the PIC didn't kill the computers the plane would've been fine all the way to the airport. No need for a plane upset.

D_Bester Granted, pulling those CB's was not a good idea, but even without the computers the plane was entirely flyable. It caused the a/p to disconnect and control laws to downgrade, but even withoutb the FAC there was no need for a plane upset. The upset was caused by pilot input. Other than the cause of the a/p disconnect this was almost a carbon copy of AF447.

According to the official investigation report, both the flight crew on this flight had received stall recovery training, but not upset recovery training, on the A320. The Flight Crew Training Manual for the A320 (issued by Airbus) did not mandate upset recovery training. The operator's training manual (issued by Air Indonesia) did mandate upset recovery training, but it was not delivered by the airline for pilots of the A320 because of Airbus's statement that it was unnecessary. Only upset prevention training was given. Both the below are from the linked report:

The  approved  Operation  Training Manual covers  the  upset  recovery
  training  in Chapter 8. The  module  consisted  of ground and simulator training. The ground training provides the flight crew with the background, definition, cause of  aircraft upset, aerodynamic and  aircraft systems in
  relation with aircraft upset. Recovery methods consider various aircraft attitude and speed including post upset conditions.
The  upset recovery training  had  not  been  implemented on Airbus  A320
  training, since it  is  not  required  according  to  the  Flight  Crew  Training  Manual  and  has not been mandated by the DGCA.

and later

The Upset Recovery training was included in the aircraft operators training manual. The aircraft operator advised the KNKT that the flight crew had not been trained for the upset recovery training on Airbus A320, and this referred to FCTM Operational Philosophy: “The  effectiveness of fly-by-wire  architecture,  and  the  existence  of control  laws,  eliminates  the  need  for  upset  recovery  maneuvers  to  be  trained  on
  protected Airbus”. There was no evidence of DGCA findings for this incompliance of training.

Upset aircraft is where the attitude or airspeed is greatly outside the normal flight envelope, such as might happen after a mid-air collision or loss of control, or (as in this case) prolonged wrong inputs when fly-by-wire is disengaged. A stall is not necessarily an upset aircraft condition.
The report also notes that the stall recovery training doesn't necessarily cover the developed stall, but rather recovering when the stall warning sounds before actually reaching the stall. The exercise is carried out from level flight by reducing airspeed and pitching up progressively to trigger the stall warning, and then pitching down to recover. (This is the same way stall recovery is taught to private pilots in smaller aircraft.)
Recovering from a developed stall where the aircraft's attitude is level but it is descending rapidly (stall at zero pitch) is not mandatory training (p119), and this is the situation the flight crew were unable to recover.
As there are too many crashes resulting from unrecovered stalls, the whole industry has worked together to produce better training programmes. One of the Safety Actions resulting from this crash is that Air Indonesia now gives upset recovery training to all its pilots, to comply with its own policy. In addition, the airliner manufacturers have produced a new training aid (applicable to most airplanes) for teaching upset recovery, and Airbus has already issued an Operations Training Transmission with instructions for carrying out suitable simulator training.

I guess the poor aircraft would be pretty upset at that point........

@kevin Is that in the sense of "you're pretty when you're upset"?


text:
What is the legal situation if a passenger dies in mid-flight?
Reading this question, a new one came to my mind. That question is explaining how an airline behaves in that unfortunate situation. However, what are the legal consequences?
Concretely I would like to understand the following situations?:

Only a doctor can certify the death of a person. If no doctor is present in the flight (or all doctors have been dragged off due to overbooking), how is the death certified? On the ground I guess.
Can the passengers leave the airplane or they will need to wait until the authorities get inside?
Which country will be liable for any potential criminal investigation (if applicable)

This is probably better on Law.SE

See "Tokyo Convention" (ICAO) which may be relevant. The full text is here.

The latest ICAO recommendation that I could find is at https://www.iata.org/whatwedo/safety/health/Documents/guidelines-death-on-board.pdf. It recommends disembarking passengers first, and that is what we did on Hajj flight in the 1990s when there was a death on board.

Re "or all doctors have been dragged off due to overbooking" --um, wouldn't this be redundant with "If no doctor is present in the flight" ?

Unfortunately, having been on board in this situation I can tell you what happened on board my flight:

The deceased was covered and secured in the seat.
The passenger sitting next to the deceased were relocated.

Needless to say, it was a very somber flight. No announcements were made to the effect, but for the people sitting around the rows, it was very sobering.
The aircraft landed and everyone was told to remain in their seats. ER staff along with a bunch of other personnel came onboard.
The body was removed.
Passengers were deplaned.

Years later, I was watching an episode of Ultimate Airport Dubai where an Emirates flight suffers a similar tragedy where a business class passenger is found unconcious.
In addition to the normal scenario on board that I faced, the documentary revealed that:

The crew was changed
The person was pronounced dead by the local authorities


text:
Which UK airport uses different approaches for noise reduction?
I remember hearing about a British airport that had two approaches that were used on alternating weeks. They implemented this pilot project (no pun intended) so that different houses would be overflown each week, giving residents the impression that noise from the airport was reduced. What airport was this?

Apparently it's called Runway Alteration. The only hits I got were referring to Heathrow airport, so my guess is this is the only airport that uses this procedure.
Actually, the alternation is in two ways: per part of the day, and per week.
During week 1, from 06:00 to 15:00 runway 27L is used, and from 15:00 until last departure 27R is used.
The week after this, the things are reversed:
From 06:00 to 15:00 runway 27R is used, and from 15:00 until last departure 27L is used.
So the people living under the approach of 27L will have noise in the first part of the day in week 1, and during the second part of the day in week 2.
See the full programme here.
It is influenced by the six factors below (taken from the same source):

Daytime patterns of runway alternation are different to night-time    patterns: at night there are only a handful of landings and very few take-offs.
Aircraft land and take off    into the wind, so the wind direction has a big effect on flight patterns.
Government policy currently favours aircraft taking off    towards the west. Even when the wind blows from the east, aircraft    could
  still fly in from east and take off towards the west. This is because we have a westerly preference.
A historical restriction means    that aircraft cannot take off towards the east from the northern    runway. This is because of the Cranford Agreement.
The hour between    06:00 and 07:00 is the busiest time of the day for arrivals so we are   able to use both runways for landings.
Occasionally we have to suspend    runway alternation for airfield maintenance, in response to bad    weather or when air-traffic control
  want to avoid a build-up of    arriving aircraft.

Actually, alternation also occurs for night flights, but then in 4 one-week periods.

Take-offs from Heathrow are permitted with up to a 5 knot tailwind component (from memory) so runway changes due to wind direction changes are minimised.


text:
May I fly an aircraft registered in an EASA member with an EASA licence (NOT same country as airplane) outside Europe?
Let's say that I have an Italian PPL licence and with my I- registered aircraft (small Cessna example) I want to fly in the US or in Argentina, then ICAO rules allow me to do that. Am I correct?
(I think ICAO rules let you fly in another ICAO country if you have your licence and aircraft from same country)
Assuming that, would I be able to do the same thing if my aircraft is registered in another EASA country like Germany (D- registration)?
And a final question:  What if instead of PPL and Cessna, we put EASA glider licence and a glider?

Welcome to Aviation.SE maximus. Interesting question!

Under ICAO rules, if Germany 'validates' your Italian license then yes, you can fly the German aircraft in other countries:

Use of flight crew licences on foreign-registered aircraft
Any pilot
who wishes to fly on an aircraft registered in a State other than the
one that has issued the licence, needs to obtain an authorization from
the State of Registry. This authorization is generally given by the
State of Registry through a validation or a conversion of the foreign
licence. In general, the validation process is used for short-term
authorization while the conversion process is used for longer-term
authorization.

What 'validation' means here is up to the country where the aircraft is registered. ICAO itself doesn't define what they should do:

When a State validates a foreign licence, it recognizes it as valid
for use on aircraft on its own registry. The Convention on
International Civil Aviation and its Annex 1 do not contain specific
requirements for the validation of licences beyond establishing the
principle and the fact that the validity of a validation, cannot be
extended beyond the validity of the supporting licence.

In the US specifically, these ICAO rules are implemented in 14 CFR 61.3 (emphasis mine):

(b) Required pilot certificate for operating a foreign-registered
aircraft within the United States. No person may serve as a required
pilot flight crewmember of a civil aircraft of foreign registry within
the United States, unless—
(1) That person's pilot certificate or document issued under §61.29(e)
is in that person's physical possession or readily accessible in the
aircraft when exercising the privileges of that pilot certificate; and
(2) Has been issued in accordance with this part, or has been issued
or validated by the country in which the aircraft is registered.

Practically speaking, I would say that if you have some official piece of paper from the German authorities confirming that you are authorized to operate German aircraft - or at least this German aircraft - then you're good to go. The piece of paper may even be unnecessary as far as Germany is concerned, but if you plan on flying internationally then I think the more pieces of official paper that you have, the better.

Understood, so if I have an Italian licence, a German plane and want to fly in US I need a "validation" of my italian licence from Germany, but not if I fly for example in Spain since all (licence, aircraft and country of flight) are within EASA "zone". This is crazy haha!!

@maximusboscus Yes, that's my understanding of the ICAO information. ICAO doesn't define "validation" so it's possible that Germany has defined it as "if the license comes from another EASA country it's automatically valid". For a reliable answer, I would ask both the German and Italian authorities.

Update for anyone landing here in 2018 or later: new licences are now automatically validated between different member states of EASA. These licences contain a phrase that references the following document: https://www.easa.europa.eu/downloads/46139/en

@TimoKluck this is indeed very good news. Especially now that national glider licences are all being converted to EASA SPL.

By treaty, if you are flying Italian-registered aircraft you cannot be legally denied to fly virtually anywhere in the world, including the United States. Some countries will try to bully you into taking tests, especially for instrument flying, but whether this would hold up in court is dubious. Treaty law is very powerful, at least in the United States, and in a US Federal Court, if you can cite treaty, it completely overrules any FAA "regulations". For this reason, it is EXTREMELY rare for the FAA to question a foreign pilot's credentials, because they know if it went to court they could get their ass kicked for treaty violations, which is taken very seriously by Federal judges.
If you are flying a foreign-registered aircraft (not your own country), you need to have a license from that country (in this case Germany), or from the United States. You can "convert" your Italian license into a US one, but the process is a pain in the ass and takes several months.
For gliders it is the same deal as a powered aircraft. As long as it is an Italian glider you are good to go. For US gliders it completely depends on where you are getting the glider from. For example, at our club there is a French guy who is an expert glider pilot, but the instructors at our club will not let him fly the club gliders. They are giving him check ride after check ride. He spent the whole summer in "check rides". It was a joke because he was a better glider pilot than a lot of the instructors. He probably would have been better off going through the hassle of getting a conversion.

Tyler thank you for your answer. When you write “For US gliders it completely depends on where you are getting the glider from” are you saying it depends on which US state you are getting the glider?? I thought FAA rules are independent from (US) states.

Second question: So implicitly you are telling me that the fact that I can fly within all EASA countries regardless of my licence country and aircraft registration country (always EASA) DOES NOT apply outside.
This begins to ressemble to this question and this confirmation from the FAA.
Are my assumptions correct?

It has nothing to do with the state. If you are renting a glider, it depends on who you are renting from. Some clubs will happily rent you a glider no questions asked. Others (like ours) will want to give you a colonoscopy before letting you get in the cockpit.

(Second question) Right, in the US you either need to have a (1) US license, (2) a conversion, or (3) your license must match the registration on your aircraft. The terms of the treaty only apply to foreign pilots flying aircraft registered in their country of origin.

@TylerDurden That's not quite correct: 61.3 says "issued or validated" by the country of the aircraft. So as long as Germany "validates" his license, it doesn't have to actually be a German one.


text:
Would a 19,000 ft descent in 6 minutes feel noticably different to passengers?
In July 2008 a Qantas 747 suffered decompression at 29,000 ft and the pilots then descended to 10,000 ft in the space of six minutes . More details here : wikipedia.org/.../Qantas_Flight, 30 .
I'm curious to know if you're in a large aircraft descending at that rate would there be a strong sensation of descent for the passengers? Would the plane be angled downwards in a way that wouldn't normally happen or some other sensation not normally experienced when travelling on large civil aircraft?
I appreciate that in this case the masks were down and there might have been noise/wind associated with the actual incident, so clearly it did feel strange to these passengers, but if the descent was done as a test at that rate would it feel strange?

Yes, it would be noticeable. 19,000ft in 6 minutes is roughly 3166 feet per minute, or ~53 feet per second. Noticeable, although not overly aggressive if that is what you are asking.

@RonBeyer  3000 fpm is a standard descent rate in a transport category jet.

@casey Right, it's a noticeable decent rate but not aggressive, I didn't suggest it was outside normal procedures.

From the physical side, you can not measure / feel if you are at a constant altitude or if you are ascending / descending with constant speed. What you can feel is the change of speed in any direction, and acceleration acting in an unexpected direction. This causes the usual sensations:

Accelerating / braking on the runway: You are pressed into the seat / belt.
Weird feeling right when the aircraft becomes airborne: The aircraft accelerates upwards, which presses you into the seat with more than 1g. This higher g-force feels strange.
While climbing, the aircraft still accelerates, but the more strange feeling comes from the fact that the force doesn't point down to the floor of the aircraft, but more backwards. Your eyes see that you are sitting upright in the aircraft, but it feels more like lying.
At some point, usually when the flaps are retracted, the climb rate decreases suddenly and it feels as if the aircraft falls. This is because the aircraft accelerated downwards - tough it still climbs, but at lower rate.
Usually, you don't feel much during a coordinated turn. Maybe you feel a little heavier, but when you look out of the window and don't see horizon, this again gives a strange feeling.

Of course, the passengers of that flight did have several sensations. right after the incident, the aircraft for sure has done some unusual movements, plus what the pilots did.
The reference #7 of the Wikipedia article contains this diagram on page 36:

Directly after the incident, the aircraft climbed a little and the speed brakes were deployed (50%) to reduce speed. This was for sure noticeable by the passengers. And I guess the aircraft started to descend faster than usual, so the sensation of "becoming lighter" will have been much stronger/longer than usual.
After this, the aircraft descended with a quite constant rate for several minutes. This doesn't cause the feeling of being lighter. Maybe the attitude was unusual which means gravity doesn't push you vertically into the seat.
Later, when the aircraft reached 10,000ft, it accelerated upwards to stop the descent. At this moment, the passengers felt heavier.
(If you have a closer look, the descending part isn't a perfect straight line. This wasn't a smooth ride, but as said, a constant descent rate doesn't cause a sensation)
Of course, this is just about the sensation of acceleration so far. The passengers for sure have noticed the drop in cabin pressure, drop in temperature, more noise and so on.

I would imagine it felt a bit bumpy though due to the speed brake, like driving on gravel.

Yes, it would feel like an elevator descending very quickly. Elevators in small buildings generally descend at around 500ft/min. As an aviator you're taught that this is a comfortable descent rate for almost everyone, and it's what most General Aviation (GA) pilots shoot for. Elevators in most skyscrapers operate up to 2000ft/min. At 2000ft/min a human body reaches roughly one-fifth of its free fall velocity, making a person feel 20% lighter. This is a noticeable and sometimes nauseating effect. The other way that people feel descent rates is through pressure changes, often making ears pop and hurting young children. This isn't as much of a factor for commercial flights as their pressure changes only slightly above 10,000ft MSL.
I mentioned that most GA aircraft ascend/descend around 500/ft min for comfort. Most turbine or commercial aircraft will ascend/descend on instrument clearances that call for rates of 1000ft/min - 1500ft/min.
The pilots in the Qantas case did a great job. They achieved the perfect glide slope for that aircraft and conditions. Gliding that plane for 20 minutes would have resulted in a splashdown for most pilots.
Anectdotally, I am a pilot and have descended at 3000+ ft/min and can tell you it is VERY disconcerting if you're not conditioned for it.

Actually, you don't feel lighter when ascending/descending with constant rate. in a lift, you feel heavier when it accelerates upwards, then feel your normal weight while moving with constant speed, and then lighter, when the lift stops. You may notice pitch and of course change of air pressure, but from the movement alone, you can only notice change of speed in any direction.

Welcome! "This isn't as much of a factor for commercial flights as their pressure changes only slightly above 10,000ft MSL" No sure this is correct: What is the pressure in a civil aircraft fuselage at flight ceiling?.

I don't agree with parts of this answer. In an aircraft, descent rate is relative, descent angle is what is important. A few minutes on Flightradar24 confirms that at the top of descent (and importantly, at near-cruise speed) a rate of -3000ft/min is common in airliners, giving a descent angle of circa 3 degrees. But as the speed slows for landing, the descent rate also lessens in order to maintain the same 3 degree angle. The passengers don't really notice the change in descent rate, they just notice the angle. I certainly wouldn't want to do -3000ft/min in a Cessna at 80 knots!

This answer seems to be suffering from a confusion of speed and acceleration. 3000ft/min descents are reasonably common for commercial airliners and pose no problems whatsoever - after the initial 'push over' there's no feeling of 20% lighter at all.

@sp1: This would be true if the aircraft did a perfect linear descent (no wind), else centrifugal force is perceptible and depends on the speed.

@mins That's incorrect. The presence of wind is not going to affect ones' perception of a descent. Also, the perception doesn't depend on the speed once the initial push-over is done - as I said.

As the above states you only feel acceleration, not constant motion. It is VERY disconcerting that you are a pilot and have not grasped this basic concept.


text:
Are these pressure and velocity distributions calculated by CFD simulation correct?
I am simulating wind tunnel tests for a NACA64210 airfoil with the following conditions:
$P_\infty=101300$ $Pa$ ; $\rho = 1.23$ $kg/m^3$ ; $U_\infty = 29.99$ $m/s$
I performed the simulation with Solidworks Flow Simulation, and I obtained the following plots.

My question is about if these plots make sense and can be correct.
For instance, I am wondering why in the velocity plot, the velocity is not zero at the leading edge (provided it should be a stagnation point), maybe it's because the wind tunnel speed is low.
On the other hand, as far as I know, the pressure distribution in a wing should be smaller in the upper side of the wing than in the lower to successfully create lift, in this case it seems to be equal at both surfaces. Also, there's usually a suction closer to the leading edge that I cannot detect here.
I just need a review of my plots to know if they are correct or not, and if they are not correct what might be the cause of it.
P.S: The only setting that I was not sure about what to input, was that I enabled that the air was laminar and turbulent. I'm not sure if I should have stated only laminar, instead.

The stagnation point is a mathematical concept, in reality, the velocity will not be exactly zero. However, it should be very low.
I would advise you to determine the Cp, Cl and Cd, and see if they make sense.This will also answer your question about the pressure above and below the foil.

Qualitatively they seem to be correct:

Highest pressure at stagnation point and maximal pressure in diagram close to total pressure:
$$101843.84\ Pa \approx P_{tot}= P_{\infty}+\frac{\rho}{2}\cdot U_\infty^2 =101853\ Pa$$
Speed drop at stagnation point.
The slight flow deflection downwards behind the airfoil shows that your airfoil is generating lift (I am assuming your free stream flow is in x-direction).
Low pressure (dark blue) region above airfoil is bigger than below.

As @ROIMaison wrote the correct way to validate the results is looking at the aerodynamic coefficients and pressure ditribution $c_p$. In a quick search I found some polars calculated with xfoil you should be able to replicate. With xfoil you can calculate the pressure distribution too.
If you are only interested in lift at small angles of attack laminar flow simulation should be enough. If you also want to calculate drag I would set turbulent air. To validate your setup (model, boundary conditions,...) for turbulent flow there is a great NASA webpage.


text:
Which are typical rpm values for aeronautical turbines?
I wonder which of these has higher or lower rotational speed:

turboshaft
turbofan
turbojet
turboprop

Does anybody know the typical speeds for these kind of engines?

The same engine core is often used as all of turbofan, turboprop and turboshaft, so that makes no difference. What makes difference is speed (smaller engines run at higher rpm).

Some figures in this answer: Is it possible to drive a turboprop directly from a jet engine without a gearbox?

First of all: There is not one single speed on many aircraft engines. Big propeller engines have a reduction gearbox so the propeller runs at a lower speed than the core engine, and multi-spool turbines have an inner, high-pressure section with higher rotational speeds than the outer, low-pressure parts of both the compressor and the turbine. Take the Progress D-27 turboprop for example: It has three spools and a reduction gear, so its components run at four different speeds.
Generally: The bigger the diameter, the lower the typical speed. Fan and propeller blades should run at a high dynamic pressure, but still at subsonic speeds to avoid the higher losses involved with supersonic flow. Since the tip will be the part with the highest speed, and propellers have bigger diameters than fans, propellers run at lower speeds. Also, their bigger diameter makes propellers more efficient. The more air mass is involved in thrust creation, the higher the propulsive efficiency will be. Propellers on slow-flying electric or human-powered airplanes run at only hundreds of RPM, well below speeds where their tips would approach Mach 1, in order to convert as much of the limited power to thrust as possible.
If flow speed allows, core engine parts run at the highest RPM possible, because the faster a turbine or a piston engine runs, the more power it will produce for a given size. Therefore, core engines of the same size run at the same speed, regardless of their type, their bypass ratios or their use. Only stationary gas turbines for power generation might run a bit slower than their aircraft-mounted siblings to increase their life.
The high pressure compressor and turbines of an airliner engine will run at above ten thousand RPM, while the propeller of a large turboprop engine wants to run at about 1700 to 2200 RPM, and in case of the NK-12 of the  Tu-95 the propellers run at only 750 RPM. Both the fan and the propeller are powered by separate low pressure turbines. The fan and low-pressure compressor of a big airliner engine run at between 2500 and 4000 RPM, and the turbine of the PT-6, a classical turboprop engine, at 30.000 RPM. Since power is torque times RPM, a faster running shaft will be lighter than one running at low speed, so the speed is only reduced directly at the propeller mount.
A fan's optimum speed is between that of the compressor and the propeller and would ideally also need a gearbox, but here the transmitted power is so high that a compact gearbox will be hard to design. Even if it runs at 98% efficiency, it will produce waste heat in the order of several hundred kilowatts in case of a big airliner engine. Some small engines with just a single turbine use gearboxes already (e.g. the Lycoming ALF507), and now the next generation of efficient fan engines are also introducing gearboxes. But so far, most engine designs have preferred to run the fan faster than what would be ideal. Note that the tip speed of a modern, ungeared fan is already well above Mach 1.


text:
Do small airplanes have AC power?
I know that DC power is essential for avionics and that in medium and big airplanes AC (28 V, 400 Hz) is used with DC (28 V).
Small airplanes have a starter-generator that provides DC, but do they have an inverter too, to provide AC? If yes, then why use AC, and in which systems? And when is AC 26 V used?

Related: Why do airplanes use AC electric power?

The single-engine Cessna planes I've been checked out on (172P, 172RG) have no A/C electrical system.

As far as I know, all electricity is AC at first. All generators and alternators initially generate AC, which can then be converted to DC. This is because of the rotating motion of the coils or magnets in the generator.

@Sami: Yes, but in DC motor and DC generator the conversion is done by commutator directly on the shaft, so there is no AC outside of the motor/generator.

All the light planes that I know of have electrical systems almost identical to automobiles.  In fact, the alternator on my 60s-vintage Cherokee said "MoPar" on it.  And what on Earth would you use A/C for, anyway?

Ford/Prestolite in older small Cessnas.  Not many generators left, most have alternators now, either designed in or converter over.

I have worked on large and small aircraft.  You will find a 400hz power bus only on large aircraft. The reason is the greater quantity of poles in the generator and hence weight. Smaller aircraft use a 28v DC system for the avionics.

What about AC 26 V? When is it used?

26VAC is used on some analog instruments. Especially ones that use air. I've seen 26VAC on instruments like gyros etc ...

And to answer a second part of your question the 26vac is provided by either a static or analog rotary inverter. They are just small boxes.

For small airplanes as you said it is used DC generation mainly to feed the battery which provides energy for the starter generator. I do not know of such airplanes having DC generation and the inverter for AC but I know that the ART-42 uses double generation DC and AC as well.

Sorry to disappoint you, but avionics (like all computers) use DC for sure.

Honestly it is reasonable...otherwise in an emergency situation main systems could not be feed by the battery right ?


text:
Which kind of display are used on commercial aircraft?
There are many kinds of display (LED, LCD, PDP...).
Which type is used in the in-flight entertainment system on commercial aircraft?
And which kind of display is used in the electronic flight instrument system?

Related discussion

Traditionally, it was almost only movie projectors in the cabin, since Cathode Ray Tubes (CRT) were too bulky and impractical to fit. It was also very expensive. I think only with the arrival of LCD displays seatback displays that we know now were made possible.

As for cockpit displays, the CRTs were widely used until LCDs were introduced and offered better performance.

You can compare the 737 Classic with retrofitted CRT displays with those of the 737 Next Generation.
Compare the 747-400 to the 747-8 which had LCD in place of CRT.

I can't find pictures, but have been told that the old CRT modules were huge, very deep (which you can't see) and heavy, compare to the thin panel that now is available.
You can still backwards order CRT for aircraft. I was flying with one airline that had done on a very new A320 to maintain commonality with the rest of the fleet. The pilots didn't appreciate it though, since it was bad for the eyes, needed to be dimmed and had (I believe) inferior resolution.
Page 9 of this document gives some details on the display type tradeoff.

The CRT PFD/MFD units in the EMB-145 are quite deep.  Its been a long time since I saw one being replaced, but somewhere around 1.5 feet deep or more.

I ... have been told that the old CRT modules were huge, very deep ... and heavy ??? When were you born, 2013?

^ ... that's a joke. Nice answer by the way.


text:
Is a powerback allowed by airlines as a safe maneuver?
I watched some videos on YouTube where the pilot used the thrust-reverser for push-back instead of the push-back tug. Some examples:
MD80 Powerback
DC9 Ground Reverser Powerback
Is this procedure currently in use? Would an airline today certify a 737, or even turboprop, like a Beechcraft 1900D, to execute such a maneuver?
I've also seen a Boeing 727 video. 727s and MD80s have the engines attached directly to the fuselage, not in the wings. Would that be one of the reasons?

American Airlines used to use it frequently circa early 2000's. I don't know if they still do it. I believe Crandall was trying to save money on ground crews. Crandall was as asshole; and I am glad he is gone. He ruined a lot of people's lives by turning careers into part time jobs.

Most modern commercial airliners are prohibited to power back.
Ground operations in aircraft fitted with high-bypass engines are usually restricted to idle and low-idle operations (enough to make the craft start moving, after which momentum enables further movement with idle only).
Aircraft capable of power back are predominantly thus turboprops, several operators of these allow their crews to operate accordingly.
It's not in common use anymore, for the following reasons:

Thrust reversing can kick up a lot of debris, posing a safety hazard. This would generally preclude any ground crew or spotters walking under the jet as is often done during pushbacks.
The previous point's especially true for under-wing jet engine designs, so only jets with fuselage-mounted engines are allowed to do it, and most of the bigger designs (DC-9, MD-80, 727) are being phased out of most fleets.
Pilots in most craft can't see behind them, another possible safety hazard requiring multiple spotters to be in contact with the pilot.
Thrust reversing uses a lot of fuel, so it's usually cheaper for the airlines to use a guy on a tractor for an assisted pushback.
Thrust reversing is loud, and during a powerback that noise is in close proximity to the terminal. The tarmac's a noisy place, but usually not that loud.

If you want to see a powerback performed by a commercial airliner, fly out to L'Esperance in St. Martin. The runway there is very short and has no taxiway leading to the end of the runway from either end, so larger passenger turboprops that service that airport have to turn onto the runway about 2/3 down its length, face their desired takeoff direction, then powerback to the end of the runway in order to have enough to take off. The larger and (relatively) busier Princess Juliana International, on the Netherlands side, has taxiways and turnaround areas at both ends of its runway.

That was what I wanted! For the aviation exchange team: POWERBACK != TAXI BACKWARDS

@YgorMontenegro please refrain from shouting.

Any aircraft with thrust reversers or reversible props should in theory be able to execute a powered pushback, FOD and ground facilities damage notwithstanding.  In a previous life I was an AC-130 crew chief -- loved the fact that those things could back up.
But I think the main reason you don't see many air carriers using their thrust reversers for pushback is because of the hazard to everything outside the aircraft -- people, baggage carts, fuel trucks, jetway bits, and the terminal windows.  I can't find a reference for it at the moment, but I do remember one US airport banning powerbacks in the late 70's or early 80's after a gate area window was shattered.  My recollection is that powerbacks fell out of favor and we started seeing tugs used everywhere around that time.
Here's a good ASRS article about not only powerbacks but blast damage incidents in general.

Actually, the biggest reason why it's not used much anymore by aircrarriers is fuel burn is excessive.

I saw tons of powerbacks in the mid-to-late 90s. It seemed like MD-80/MD-90/DC-9 aircraft did it more often than not (at least here in the U.S.) back then. It wasn't until the 2000s that they seemed to start falling out of use.

An air carrier must also have OPSPEC C065, POWERBACK OPERATIONS WITH AIRPLANES to do this.

OPSPEC C065—POWERBACK OPERATIONS WITH AIRPLANES.
A. General. C065 authorizes the use of powerplant reversing systems
  for rearward taxi operations. Before issuing C065, the principal
  operations inspector (POI) must determine whether the operator meets
  requirements discussed in AC 120-29, Criteria for Approval of Category
  I and Category II Weather Minima for Approach, current edition.
  Airplane types’ make, model, and series (M/M/S) authorized for
  powerback operations must be listed in C065. Airports where powerback
  operations are authorized must also be listed. If the POI and/or
  operator determine that restrictions to powerback operations are
  required at certain gates or ramp areas, the restrictions must be
  described (adjacent to the airport name) in the “Restrictions and
  Limitations” column. OpSpecs worksheets provide a template for listing
  authorized airplanes, airports, and restrictions.
B. Policies and Procedures for Ground Personnel During Ground
  Operations. Title 14 CFR part 121, §§ 121.133 and 121.135, part 134, §
  134.23, and part 135, § 135.21, require certificate holders to prepare manuals setting forth procedures and policies that must be used by
  ground and maintenance personnel in conducting their ground
  operations. Sufficient procedures must be established to maintain an
  adequate level of passenger and company ground personnel safety during
  ramp operations. Procedures should emphasize safety during boarding
  and deplaning of passengers or cargo, specifically during times when
  an engine(s) may be running or a propeller(s) is turning during ground
  operations. Procedures should include, as a minimum, a means for
  defining no access areas around the propeller(s), as well as the
  landing gear and tugs, during push and ground marshaling operations.
  Policies should provide that an adequate number of ground personnel
  are assigned to ensure safety of company personnel and passengers.
C. Pushback and Ground Marshaling. Procedures for pushback and ground
  marshaling activities should be clearly defined and should include
  safety precautions and signals, and should ensure adequate visibility
  of assigned personnel during the time of aircraft movement.
D. Increased Awareness. FAA air carrier surveillance programs should
  emphasize increased awareness by inspectors and the strict need to
  follow safety procedures around turning propellers, in marshalling and
  pushback procedures, and/or other ground activities.
E. Other References. Additional references can be found in National
  Transportation Safety Board (NTSB) Recommendations 91-297, 91-298, and
  93-146, and Air Carrier Operating Bulletin (ACOB) 8-94-2, Safety in
  Ground Operations.

A plane can powerback if you can configure the engines to provide more backwards thrust than forward thrust.
Fixed pitch propellers can't provide any reverse thrust. With variable pitch props it depends on whether the there is enough pitch control to reverse it.
However powerback has the risk of debris getting into the engine. The air will blast forwards and possible damage the gate so it's not used there. That and the guys on the ground don't appreciate getting blasted.

I've got a few hours in the 1900 and a couple hundred in the smaller KingAir. I think you can reverse the props while sitting still and push back with the negative thrust. But you probably shouldn't. You'd probably be safer doing it in the Beech than a 737, since the intakes on the Beech are significantly smaller (and have the prop directly in front of them pushing the air/debris outward). The 737 engine doesn't change the direction that it's sucking the air in. Just how it's blowing it out. I'd still think it would be a FOD nightmare.

Both power back and taxiing under thrust are on their way out. Efficiency is horrible, so electrically powered wheels are planned for the next step in fuel efficiency improvement.
Not to be misunderstood: Currently this is not yet being used on production aircraft. But it looks likely to come.

This is probably a topic for a whole 'nuther question that would be closed due to the speculative nature of it, but are they really finding a way to electrically power the wheels with motors that are light enough to carry for the whole flight, thus making the efficiency of the whole system better than using some fuel for taxiing?

@FreeMan There is at least one company out there trying to design some sort of automated tug that would taxi it out to the runway, IIRC. To me, though, it seems like maybe taxiing out under power isn't such a bad idea, since it seems like you'd be more likely to detect any potential problems with the engines before takeoff that way.

FWIW, Delta Airlines also evaluated the possibility of using supertugs at ATL to for maintenance-related ground movements. I don't think the results of the simulation were actually published anywhere, but I've seen them in person and they indicated that, for the assumptions under investigation, this would have been an NPV-positive initiative. Can't rightly say whether this is something that would have also turned out positive for taxi-outs and -ins as supertugs are quite fuel-hungry and expensive in their own right, but the approach may be a viable third option, at least at major airports.

@reirab just taxiing out is not always likely to reveal any serious engine problems due to the low power or thrust settings usually required for the task, which is why engine run-ups are so important. Though I will grant you that they would reveal the rather embarrassing problem of not being able to actually start the engine before reaching the runway :)

Here's some info about eTaxi. http://www.lufthansa-technik.com/etaxi


text:
which part of aircraft maintenance or inspection that could be automated to avoid maintenance error?
According to my search:

Almost 12% of all aircraft accident reports cite a maintenance factor.
source

What is the actual problem you're trying to solve?

i want to create a solution to automate the process if that is needed

I have the feeling that you are trying to fix a problem that doesn't exist. As it is currently stated, your post assumes a problem: The visual pre-flight inspection (walk-around) is not accurate. It also suggests an improvement: Remove inaccuracies by automation. And your question is: Is this automation possible? To answer, it is necessary to understand what inaccuracies you are trying to remove (if possible provide examples of incidents that did occur). Could you consider editing your post to add this required information?

If the question can be re-worded, I think it could be discussed. I find it interesting from the point of view that a machine carries out the maintenance rather than a human.